Originally Posted by GordMay
To enlarge a hole (as previously noted), use two hole saws on one arbor. Use one smaller hole saw the size of the original hole, and a larger hole saw the size of the new hole.
Install the two hole saws, the smaller inside the larger. The small one should protrude at least 1/2" beyond the larger one. Insert the smaller saw into the old hole, and carefully guide the saw through the hole. Keep the drill on a straight, steady course as the larger saw cuts the new hole around it.
See also ➥ Hole Saw Tips and Tricks
➥ Enlarge Holes With The Starrett Oops Arbor | Toolmonger
Hole Saw Alignment Guides
➥ Hole Saw Alignment Guide
To me, the real advantage to doing it this way for this application is that it will help you to drill the hole plumb, which is the tricky part of drilling a hole for a 'ducer. If you use this method it will drill the hole at exactly the same angle as the original with minimal effort. This is important, if you just drill the hole square to the hull
you will obviously have serious problems. Being even a little off of plumb can cause you to have to grind for a plumb fit on the 'ducer, which results in a sloppier fit and more potential for movement and leakage. I do a lot of this, because the newer plastic 'ducer fairings are a PITA to cut perfect, and they cost about $200 a pop. I seem to be the only one in the shop who doesn't ever cut it backwards. So I get to do most 'ducer installs in the yard. Careful measurement with the bevel gauge and a level is key.